1. Field of the Invention
This invention relates to electrochemical cells such as fuel cells or electrolytic cells, particularly chlor-alkali electrolytic cells utilizing air or oxygen depolarized cathodes.
2. Description of the Prior Art
Electrodes for use in electrochemical cells, particularly air or oxygen diffusion cathodes for use in chlor-alkali electrolytic cells which are based upon carbon in combination with wetproofing agents such as polytetrafluoroethylene, are subject to failure as the result of the excessive wetting of the electrochemically active hydrophilic portion of the electrode by the electrolyte. This excessive wetting is often partially counteracted by laminating the electrochemically active layer to a porous, hydrophobic (backing) layer. The pores of the hydrophobic layer pass air or oxygen so as to make the air or oxygen available to the electrochemically active layer of the electrode.
The air or oxygen gas diffusion cathode in a chlor-alkali electrolysis cell is more energy efficient than a cathode which does not pass air or oxygen gas to effect depolarization since in diffusion electrodes the unwanted by-product (hydrogen) in the electrolysis of an alkali metal halide is eliminated. The air or oxygen diffusion cathodes not only eliminate the production of hydrogen at the cathode but force the formation of desirable hydroxide ions.
It is known to form electrochemically active layers in an electrode, particularly a cathode for a chlor-alkali electrolysis cell utilizing as an active component an electrically conductive carbon black. The tendency of the electrically conductive carbon black to be wetted by the electrolyte must be controlled within limits in order to provide an electrode with a reasonable period of usefulness. The efficiency of the carbon black active layer is reduced during operation of the cell as the pores of the carbon black become completely wet out by the electrolyte since the air or oxygen necessary for depolarization of the active layer of the electrode does not under complete wetting conditions penetrate to the active portion of the electrode so as to effect depolarization.
Prior art carbon-based hydrophilic electrolyte-active layers of air or oxygen depolarized electrodes have been rendered less susceptible to wetting by the electrolyte by blending a hydrophobic polymer such as polytetrafluoroethylene with carbon to impart hydrophobicity to the carbon and thus increase the useful life of the electrode. It is thus common to employ mixtures of carbon black and polytetrafluoroethylene in particulate form in the preparation of the electrolyte active layer of the electrode. Additionally, a homogeneous, porous, hydrophobic backing layer of polytetrafluoroethylene has been employed in prior art gas diffusion electrodes on the side of the electrode facing away from the electrolyte. In a chlor-alkali electrolytic cell, this side of the gas diffusion electrode is normally in contact with air or oxygen under pressure. Complete wetting of the electrolyte-permeable layer of the electrode tends not to take place as the result of air or oxygen being forced into one side of the electrolyte-permeable layer of the electrode, thus forcing electrolyte away from this side of the electrolyte-permeable layer of the electrode.
Heretofore, these methods of increasing the useful life of gas diffusion electrodes have not been entirely satisfactory and complete wetting of the electrolyte-permeable layer of the electrode ultimately takes place over a relatively short period of time causing failure of the electrode.
In U.S. Pat. No.4,317,789, a method of making porous strips for fuel cell electrodes is disclosed in which carbon powder present in an aqueous dispersion is coated with a binder by precipitating the binder onto the carbon powder and thereafter drying and crushing the precipitate, wetting the powder with a liquid and calendering the wetted powder to obtain the desired porous strips. An example is provided of the use of the porous strip in the preparation of a two-layer electrode for use in a hydrogen-air fuel cell. The second layer of the electrode is a barrier layer which is conductive providing electron transfer from the catalytic layer to the current distributor of the cell. It is noted that the proportion of polytetrafluoro-ethylene binder in the porous strip layer is between 20 and 99 percent and that of the carbon component between 80 and 1 percent. There is no indication in this reference that the binder is sintered as a step in the process for the preparation of the electrode strips.
As a further indication of the state of the art, Solomon, in U.S. Pat. No. 4,456,521, discloses a method of preparing three-layer laminated electrodes suitable for use as oxygen (air) cathodes in chlor-alkali and other electrochemical cells, fuel cells and in other electrochemical applications. The three-layer laminate includes an active layer or sheet containing from about 60 to about 85 percent by weight active carbon, the remainder being unsintered fibrillated polytetrafluoroethylene in intimate admixture with said active carbon. The active layer is laminated on its working surface to a current distributor and on its opposite surface to a porous, coherent, hydrophobic polytetrafluoroethylene-containing wetproofing layer.